1. Field of the Invention
The present invention relates to methods and apparatus for aligning a sighting coordinate system with the tracking coordinate system. More specifically, the present invention provides methods and apparatus for automatically tracking the reticle in a sighting system to allow the boresight coordinate system of a tracker to be slaved thereto.
2. Prior Art
In a video tracking system, such as a TV system, the video sensor is generally swept in an azimuth direction across a field of view to generate a plurality of video sweep signals. A number of these sweeps slightly offset in elevation from every other sweep may be combined to form a composite observation scene image and particular objects within that scene may be detected and tracked as potential targets. However, in tracking video from an optical site, the video return generally contains not only observation scene information but also etched, hot wire, or some other type reference reticle image. In general, the reticles so superimposed are aligned to form the basis of the tracker's internal coordinate system. One problem in such a system, however, is that the etched reticle on the optics does not correspond to the electronic reticles of the tracking system. It was thus necessary in prior systems to align the coordinate system defined by the reticles of the optics with the electronic reticles of the tracking system. In prior systems, the alignment process to make the two reference reticle systems coincide was accomplished by referencing the tracker coordinate system to the TV sweep control ramp which was initially preset to coincide with the etched reticles. While misalignments did occur utilizing such techniques, these misalignments were small and were not critical since targets subtended many sensor sweep lines and the operator could correct any misalignment errors utilizing tracer bullets.
In modern systems where lasers are utilized to guide weapons to specified targets, it is necessary that much greater accuracy be achieved. In such modern systems, a laser first illuminates a target and a weapon then senses and is guided to that illuminated region. The problem is that the laser light may not be in the sensor's (e.g., T.V., forward looking infrared, long wavelength infrared, etc.) spectrum thus making it impossible for the operator to tell the exact spot which the laser is actually illuminating as was possible utilizing tracer bullets and the like. In addition, even if very accurate periodic alignment were possible temperature drifts in the display, the TV sensor camera, and other display systems, as well as servo errors and human alignment or sighting errors would still exist. Indeed, such errors could vary significantly between the periodic alignments, thereby causing significant laser pointing errors.
In view of the fact that no pointing accuracy measure is fed back to the operator via tracer bullets and the like in these type systems, it is essential that misalignment errors be eliminated in an efficient and economical way to allow high accuracy tracking so that a specified target may be continuously illuminated. All alignment systems heretofore utilized have been open-loop type systems requiring periodic alignment. The present invention provides methods and apparatus for continuously determing the location of optical reticles to allow a closed loop feedback system to provide continuous alignment between the tracker coordinate system and the sighting coordinate system defined by the optical reticles. The present reticle locating method and apparatus also permits other functions including electronic reticle blanking and display system drift compensation.